Intermixing enables strong exchange coupling in nanocomposites: Magnetism through the interfacial ferrite inγ−Fe2O3/NiO

Abstract

$\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ particles, surface modified with NiO crystallites, form a unique nanocomposite that points to how to tune strong interfacial exchange coupling. We find that ${\mathrm{Ni}}^{2+}$ migrates into the octahedral sites of the $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanoparticle surface, and this ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$-like layer permits effective magnetic coupling of Ni and Fe sites that strengthens the interface exchange. A large increase in coercivity coinciding with a loss of exchange bias is achieved by this strong interfacial coupling that results in a ${\mathrm{Ni}}^{2+}$ moment reversal in the NiO with the $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$. This work reveals the importance of intermixing in, and possibility to use, such an exchange coupling regime to alter substantially the coercivity and hence control an important property of exchange-coupled nanocomposite magnets.